||The neuromuscular junction (NMJ), a chemical synapse formed between a motoneuron and a muscle fiber, has been widely and successfully utilized in studying chemical synapses. Indeed, NMJ is the synapse about which we know the most about synaptogenesis. In contrast to our extensive knowledge on the postsynaptic development, much less is known about the presynaptic specialization, especially the functional roles of protein tyrosine kinases and phosphatases (PTPases), which are the main focus of my thesis study. By using bFGF-coated beads to mimic muscle cells in inducing presynaptic specializations, I demonstrated that in cultured Xenopus spinal neurons, inhibition of PTPases with PV blocked both the formation and maintenance of SV clustering, but did not affect mitochondrial clustering and actin polymerization. These results suggest that PTPases balance the actions of tyrosine kinases to promote the clustering of SVs during presynaptic differentiation at the NMJ. Recent studies suggest an autocrine activation of Trk neurotrophin receptors in neurons. We tested this hypothesis in our Xenopus spinal neurons. To our surprise, we found that inhibition of Trk receptor tyrosine kinases with K252a caused quick functional and morphological changes in naïve neuritis. This is manifested by the cessation of the unidirectional axonal growth and the elaboration of an arbor of side branches. Varicosities enriched in SV and mitochondria are formed within this arbor. This suggests that downregulation of Trk signaling actually promotes presynaptic differentiation in spinal neurons.